It's all CAT 6, with a few CAT 5e jumpers, and 95% of it is in metal EMT conduit (all grounded) due to mice liking the taste of the PVC jacket.
I have thought of RFI problems, and have used some ferrite toroid cores on the free ends of the 1 to maybe 2 foot free ends of the LAN cables, no help.
If it was an RFI problem, then why in that room only and not down the hall?? As the RFI signal would have to be quite strong to affect a router.
And everything was turned off in that room as a test.
The only source for any type of high power RF signal would be or could be the cell tower I spotted the other night, but that is over 3/4 of a mile away.
And more info, the ISP tech said to try the router with the 5 GHz WIFI shut off, I did that and it works, and has worked over night, so now I know something is getting into the router via the 5 GHz section on the router, and swamping it, ooohhh I wish I had my RF spectrum analyzer....
Now putting it inside a metal box or cage may not be the solution as the antennas need to be outside any type of shielding, that would effectively negate any type of shielding that could help out.